Aircraft structures may be formed from composite materials, which are generally reinforced polymer-based materials used in place of metals, particularly in applications in which relatively low weight and high mechanical strength is desired. Composite materials are widely used in a variety of commercial and military aircraft, terrestrial vehicles, and consumer products. A composite material may include a network of reinforcing fibers that are generally applied in layers, and a polymeric resin that substantially wets the reinforcing fibers to form a binding contact between the resin and the reinforcing fibers. The composite material may then be formed into a structural component by a variety of known forming methods, such as an extrusion process or other forming processes.
Variations within composite structures (such as those of an aircraft) may be caused by a lightning strike, fire, engine exhaust, or other high temperature incidents. Incipient heat damage in composites is generally not detectable using typical non-destructive inspection (NDI) techniques, such as ultrasonic methods.
Infrared (IR) spectroscopy is an approved method for detecting variations with respect to components of certain aircraft. However, IR spectroscopy is typically used to detect variations on an outer surface of a structure. In short, IR spectroscopy is unable to effectively determine variations within a structure.
Ultrasonic systems and methods may also be used to detect variations within a structure, such as caused by heat damage. However, a structure may have already significantly changed before the ultrasonic system detects the variation.
Overall, known systems and methods for detecting variations within a composite structure, such as a portion of an aircraft, are generally not effective in detecting incipient heat variations.